<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <script src="https://cdn.jsdelivr.net/npm/vue@2.6.10/dist/vue.js"></script>
</head>

<body>
<div id="app">
    <h1>全名:{{fullName}}</h1>
</div>
<script>
    Vue.config.productionTip = false;

    const vm = new Vue({
        el: "#app",
        data() {
            return {
                firstName: "wang",
                lastName: "zhongwang",
                fullName: "",
            };
        },
        watch: {
            firstName: {
                immediate: true,
                handler() {
                    this.fullName =
                        this.firstName.toLowerCase() +
                        " " +
                        this.lastName.toLowerCase();
                },
            },
            lastName: {
                immediate: true,
                handler() {
                    this.fullName =
                        this.firstName.toLowerCase() +
                        " " +
                        this.lastName.toLowerCase();
                },
            },
        },
    });
</script>
</body>
</html>
